A gardener just installed a drip irrigation system in my backyard. He didn't installed the back-flow preventer. When I asked him why, he told me for drip irrigation you don't need the back-flow preventer! His argument was that in drip irrigation, we have to install a pressure reducer so there is no need for a back-flow preventer.

The pressure reducing valve has nothing to do with a back flow device. Back flow valves prevent contaminated water from entering the potable household water. A pressure reducing valve doesn't function in this manner.
– ojaitSep 26 '15 at 3:37

603.4.6.1 Potable water supplies to systems having no pumps or connections for pumping equipment, and no chemical injection or
provisions for chemical injection, shall be protected from backflow by
one of the following devices:

Thanks a lot for your reply. Is Check valve same as Backflow valve? I was trying to see a simplest option which can be installed in the line , but couldn't find one. Any hint are appreciated
– DavidSep 26 '15 at 3:58

2

It all depends on what code applies in your jurisdiction. In many jurisdictions including my own, the UPC applies. As you can see, a check valve is not listed as an approved device. In a residential situation an atmospheric vacuum breaker for each zone is often simplest. In a case with many irrigation zones a single pressure vacuum breaker is used, but they require protection against freezing.
– user39367Sep 26 '15 at 4:18

Yes, the example you selected is a typical residential atmospheric vacuum breaker (AVB). A key point with these is proper installation. They must be installed after the valve, above ground, usually a minimum of 6" above the highest point downstream of the AVB.
– user39367Sep 26 '15 at 5:15

Even if it isn't code were you reside it would be wise to install one for your health. But check the valves that you have installed. Many irrigation valve manufacturer's make back flow valves built into them. It's usually just above the out flow threads on the valve. Or if you shake the valve you should hear it rattle. that is the check valve that allows water to only flow in one direction.

It is also important to install back-flow preventers to discourage clogging of the drippers. After the irrigation cycle finishes, the volume of the pipe that was full of water will need to be replaced by air or it will collapse at certain points. If there is no 'vacuum breaker' type valve the air will enter the line through the dripper aperture bringing with it dirt and debris. If you want to decrease clogging of drippers and occurrence of dry-spots in the garden, I'd recommend installing such a device at the highest point of each station. If the line is very long, put one every 30 metres or so.